DONETSK, UKRAINE — A clash outside a military base ended with the first deaths so far in a Ukraine anti-militant operation as Russian President Vladimir Putin admitted Thursday in a radio interview that Russian soldiers did secretly invade Crimea to help pro-Russian Ukrainians take over the province by force.

Meanwhile, high-level talks aimed at calming soaring tensions over the crisis in Ukraine were being held in Geneva between representatives of the United States, European Union and Russia.

Three pro-Russian militants were killed in a skirmish outside a Ukrainian National Guard base in the Black Sea port of Mariupol in an attack Ukraine’s interior ministry called the bloodiest to date by militants who want the entire East Ukraine to go the way of Crimea.

Crimea was annexed by Putin after Ukraine militants took over the province with the help of mysterious well-armed squads of masked soldiers who sealed off the province to prevent Ukraine from bringing in troops. Ukraine said Putin is doing the same thing in East Ukraine, but he denied it.

“It’s all nonsense, there are no special units, special forces or instructors there,” Putin said in a call-in show in Moscow.

However, Putin admitted for the first time that the troops in unmarked uniforms who captured Ukraine’s Crimean Peninsula before its annexation last month by Moscow were Russian soldiers.

“They acted politely, but resolutely and professionally,” he said. “There was no other way to hold the referendum in an open, honest and honorable way and allow the people to express their opinion.”

Putin said Russia had to act to prevent what he said was NATO’s intention to make Ukraine a member and limit Russia’s presence in the Black Sea region. NATO, the European military alliance, had no plans to do so and Ukraine had not moved to revoke a lease it had with Russia for its base on its Black Sea port.

Putin denied charges by Ukraine that Russian secret forces have also infiltrated Ukraine and are assisting pro-Russian militants in their confrontation with the central government. The militants have taken over government buildings in at least eight cities.

But he maintained he had the authority to invade Ukraine, saying he hoped it would not be necessary.

“I remind you that the Federation Council has given the president the right to use armed forces in Ukraine,” he said, referring to the upper house of parliament. “I really hope that I do not have to exercise this right and that by political and diplomatic means we will be able to solve all of the sharp problems.”

In talking about the crisis he repeatedly referred to “New Russia,” a broad area of Ukraine that had been part of the Russian empire since the 18th century until Ukraine was joined with Crimea by the Soviet Union. Ukraine declared independence in 1991, when the Soviet Union dissolved.

New Russia is generally considered to have included all of East Ukraine as well as land in the south that includes the Black Sea port city of Odessa and Ukraine’s industrial city of Dnepropetrovsk.

The skirmish Wednesday in Mariupol came after the government announced an anti-terrorist operation to retake control of the eastern city of Slovyansk — the focal point of the armed insurgency — and government buildings in several other cities in the east.

In the attack Wednesday night, a mob of around 300 people armed with stun grenades and firebombs attacked the base, refusing to back down even after servicemen in the Mariupol base fired shots, the interior ministry said.

APTN footage filmed outside the base on Wednesday night showed an unidentified man coming out to speak to masked men in the crowd armed with assault rifles. He told them the military had asked for 10 minutes to think over an unspecified ultimatum.

The masked men insisted they wanted no bloodshed. A short while later, however, a crowd of mainly masked young men armed with bats and sticks began throwing Molotov cocktails at the base’s gate and at the trucks parked in front of it. Sounds of gunfire were heard in response.

One soldier involved in the battle, a 20-year old conscript who gave his name only as Stanislav, said troops were forced to act in self-defense.

“We were attacked by unidentified people and we didn’t want to shoot, but they were behaving aggressively,” he told the AP. “At first we fired in the air, but they continued advancing.”

One protester admitted to a hospital with a bullet wound to the stomach said soldiers opened fire on them while they were attempting to force open the gates.

“We just threw Molotov cocktails to light the way,” said Sergei Shevchenko, a 40-year-old businessman from the regional capital, Donetsk.

Residents were divided about the night’s events.

“Russia isn’t just exporting oil and gas, but also terrorism,” said 43-year-old resident Yevgeny Nechiporenko. “This shooting and blood, the blood is on Russia’s hands.”

Yet passersby berated Nechiporenko as he spoke, with one accusing him of being an “agent of the West.”

“We are willing to give up our lives so long as we don’t have to serve the fascists from Kiev,” resident Anna Govorko said.

The protests in the east have been ongoing since former president Viktor Yanukovych’s ouster in February and the Russian annexation of Crimea last month. On Thursday, Ukraine acceded to the standards of the International Criminal Court so the court would agree to investigate the killing of 80 protesters in February under Yanukovych.

Tens of thousands of Russian troops deployed just across the border from Ukraine are spurring fears that the Kremlin might use the instability in the predominantly Russian-speaking East Ukraine as a pretext for seizing more territory.

“I strongly want Donbass (the area covering the Donetsk and Lugansk regions) to stay in the Ukraine,” said Yuriy Ivanov, 49, from Donetsk. “Every revolution or other great change means a downgrade. Prices for all foods would go up.”

Others worry less about economic consequences and want to join Russia.

“Ukrainians are cowards. When troubles began, for example, our old power escaped, and their supporters changed colors,” Alexander Ryabchenko, 25, from Donetsk said.

“Donbass has to be part of Russia. We all know who created this Ukraine — Lenin, Brezhnev etc. We always were a part of Russia.”